Robust inverter topology

ABSTRACT

An inverter system includes an inverter controller and at least one surge protection device. The inverter controller is configured to operate multiple switches to provide an alternating current to an output. The at least one surge protection device is configured to limit a voltage on a direct current link of the inverter. The at least one surge protection device includes a variable resistance component coupled to the direct current link of the inverter, a sensing resistor selectively connected in parallel to the variable resistance component by a switching component, a detection device configured to measure a parameter of the sensing resistor, and a control circuit including failure sensing logic configured to detect a failure in the variable resistance component in response to information from the variable resistance component and information from the detection device for the parameter of the sensing resistor.

CROSS REFERENCE TO PRIOR APPLICATION

This application claims priority benefit of Provisional Application No.62/697,697 (Docket No. 10222-18020A) filed Jul. 13, 2018, which ishereby incorporated by reference in its entirety.

FIELD

This application relates to the field of inverters, and morespecifically, to grid-tied inverters where efficiency of conversion androbustness are high-priority considerations. This application relates tothe field of ground fault detection and management. More specifically,circuitry and techniques are provided that minimize the amount of energydissipated through a ground fault.

BACKGROUND

An inverter device may convert direct-current electrical power intoalternating current electrical power. An inverter may beuni-directional, converting direct current (DC) power from a source toalternating current (AC) power out, or bi-directional, converting DCpower from a source to AC power out as well as converting AC power froma source to DC power out to a load or storage device.

A grid-tie or grid-tied inverter is an inverter that is specificallydesigned to have the AC portion of the inverter connected to the powergrid. A grid-tied inverter may be designed to operate within a window ofoperation for the utility voltage. A grid-tied inverter may beuni-directional or bi-directional. A uni-directional inverter may beused to convert a direct current input from a source, like a solararray, fuel cell or battery, to alternating current at the appropriatefrequency and voltage to be received by the power grid. A bi-directionalinverter may be used to convert alternating current from the power gridto a direct current to be received by a DC load or an energy storagedevice, such as an electrochemical element like a fuel cell or batteryor an electromagnetic storage element, such as an ultracapacitor.

A ground fault occurs when a current carrying electrical path in acircuit comes into electrical contact with a ground path. The groundpath may be a wire (e.g., ground wire) or a portion of a grounded partof an appliance or device. Example grounded parts may include a chassisor frame of the appliance or device.

Ground faults may occur in a variety of scenarios. Damage to theappliance or device may result in the ground fault. In other examples, ahuman or animal may become the electrical path between the currentcarrying path and the grounded parts.

The response time in the detection of a ground fault and isolating theground fault from additional electrical current is vital in preventingdamage. Especially in the case of a ground fault caused by a human oranimal, the risk of electrical shock is reduced when the ground fault isquickly detected and/or current is prevented from traveling to theground fault.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are described herein with reference to thefollowing drawings.

FIG. 1 illustrates 5 seconds of response characteristics of threeexample DC energy sources with capability to produce or absorb energy.

FIG. 2 illustrates 6 milliseconds of response characteristics of thethree example DC energy sources with capability to produce or absorbenergy illustrated over a longer time in FIG. 1

FIG. 3 illustrates an example supply current waveform from a battery toa grid-tie inverter sourcing single-phase 60 Hz AC power at unity powerfactor.

FIG. 4 illustrates another example supply current waveform from abattery to a bi-directional grid-tie inverter sourcing single-phase 60Hz AC power at 0.8 power factor.

FIG. 5 illustrates the bus voltage and input current profiles for twoexample techniques of controlling a bi-directional boost converter.

FIG. 6 illustrates input current profiles for boost converters using twodifferent inductors.

FIG. 7 illustrates an example circuit for a bi-directional boostconverter containing an input filter in addition to the boost inductor.

FIGS. 8A and 8B illustrate input current profiles for boost convertersutilizing an input filter in conjunction with different boost convertercontrol techniques.

FIG. 9 illustrates an example surge suppression device.

FIG. 10 illustrates an example inverter configuration containingmultiple redundant surge suppression devices and output overcurrentprotection.

FIG. 11 illustrates a droop characteristic curve for an exampletransformer.

FIG. 12 illustrates an example system containing an inverter connectedto a transformer fed by the utility.

FIG. 13 illustrates an example current amplitude flowing from theinverter and transformer of the system in FIG. 12 with a changing load.

FIG. 14A illustrates an example inverter with output amplitudecorrection.

FIG. 14B illustrates an example inverter with closed-loop outputfeedback.

FIG. 15 illustrates an example system containing an inverter connectedto a synchronous electrical machine driven by a prime mover.

FIG. 16 illustrates example current waveforms of current flowing betweenthe inverter and transformer of the system in FIG. 14 and/or theinverter and synchronous machine of the system in FIG. 15.

FIG. 17 illustrates an example system containing a plurality ofinverters with closed-loop output feedback connected together to acommon load.

FIG. 18 illustrates closed-loop inverter behavior into a short circuitcondition.

FIG. 19 illustrates an example controller for the surge suppressiondevice and/or inverter.

FIG. 20 illustrates a flowchart for the controller of FIG. 19.

FIG. 21 illustrates an example circuit for ground fault minimization.

FIG. 22A illustrates an example output of an inverter.

FIG. 22B illustrates an example control signal for the inverter of FIG.22A.

FIG. 23 illustrates examples for the switches of FIG. 21.

FIG. 24A illustrates an example output of an inverter responsive to aground fault.

FIG. 24B illustrates an example control signal for the inverter of FIG.24A.

FIG. 25A illustrates an example output of an inverter responsive to aground fault.

FIG. 25B illustrates an example control signal for the inverter of FIG.25A.

FIG. 26A illustrates an example response by actively controllingswitches of the inverter.

FIG. 26B illustrates an example with the switches of the inverter turnedoff.

FIG. 27 illustrates an example plot of voltage or current provided to aground fault using active control.

FIG. 28 illustrates an example plot of voltage or current provided to aground fault without active control.

FIG. 29 illustrates an example plot for energy delivered to the groundfault.

FIG. 30 depicts a typical magnetization curve for an example magneticmaterial.

FIG. 31 provides an illustration of the voltage of an internal energystorage element for an inverter reduced during a short circuitcondition.

FIG. 32 provides a comparison between two different output waveshapes

FIG. 33 illustrates an example circuit for ground fault minimizationdetected by resistive impedance.

FIG. 34 illustrates an example circuit for ground fault minimizationdetected by capacitive impedance.

FIG. 35A illustrates an example system with ground fault detected at aninverter circuit.

FIG. 35B illustrates an example system with ground fault detected at aninverter circuit coupled with an engine generator set.

FIG. 36A illustrates an example system for ground fault detection at aload circuit.

FIG. 36B illustrates an example system for ground fault detection at aload circuit coupled with an engine generator set.

FIG. 37A illustrates an example system with a controller for groundfault detection at a load circuit.

FIG. 37B illustrates an example system with a controller for groundfault detection at an inverter circuit.

FIG. 38 illustrates an example flow chart for the system for groundfault detection.

DETAILED DESCRIPTION

An inverter system that allows for operation with a variety of sources,loads, and other devices in parallel operation, where the invertercontains discrete surge suppression modules which can detect aninability to protect the system and are replaceable during operation ofthe inverter. The inverter automatically detects source characteristics,load characteristics or devices in parallel with the output and selectsa mode of operation based on a variety of factors, including userconfiguration and user input. The inverter discerns different devices inparallel and chooses a mode of operation suitable for the detectedoperational mode. The inverter determines internal systemcharacteristics and provides predictive estimates allowing tighteroutput control.

A bi-directional inverter may be coupled to a power source which iscapable of absorbing and sourcing power, such as a battery,ultracapacitor or fuel cell. This power source may be able to absorbenergy at a high rate, a low rate, or a rate which is limited relativeto the previous operating rate. As an example, an ultracapacitor may beable to absorb all the energy it can store in a second without damage.As another example, a battery may not be able to absorb energy fasterthan a rate which would fully charge it in a second. As yet anotherexample, a fuel cell may be able to absorb up to its rated output powerof energy, but not be able to change the rate more quickly than1%/second from the previous energy production or absorption rate.

A grid tied inverter may be subjected to external voltage surges fromthe utility caused by environmental factors, such as a lightning strike,equipment malfunction, such as incorrect switching of a multi-tapvoltage regulator on the line or lightning arrestor failure, or humanerror, such as a collision with a power pole. These surges are appliedto the output of the inverter, potentially damaging components withinthe inverter.

While a transformer, commonly used by utilities for transmission anddistribution of power, may provide an impedance that allows the outputvoltage amplitude to vary with changes in the load current amplitude, aninverter may provide a nearly constant voltage throughout an allowableoperating range of the inverter.

As an example, an inverter may contain large, low impedances switchesand a highly-efficient output filter, causing minimal variation involtage with increasing load. An inverter may also have an outputamplitude correction mechanism to adjust for varying load on the output,further decreasing the voltage amplitude variation relative to thecurrent amplitude. Other techniques to correct output, such as capacitorcurrent feedback, modeled filter offset, and output current correctionor feed-forward will provide similar correction with variation in thereaction time to a load transient.

As another example, an inverter may also operate in a closed-loopmanner, where the output voltage of the inverter is actively controlledto a sinusoid or some other signal. An inverter operating in closed-loopmay correct for any load-related fluctuations in the output signalquickly enough that no variations are detectable in the measuredamplitude of the output. Closed-loop inverters may include currentfeedback or filter modeling techniques as well to decrease the requiredcontrol loop gains, but these techniques are not fundamentally necessaryfor this topology.

When coupled with a utility transformer or to the grid through severalutility transformers, the differences in the behavior of the inverterfrom the behavior of the transformers may result in large variations inthe power provided by the source feeding the grid through the inverterand the changes in the power provided by the utility. This is especiallynoteworthy in cases where the output amplitude is corrected, such asinverted with output amplitude correcting feedback or closed-loopinverters. An additional concern with closed-loop inverters is thatpossibility or producing harmonic currents if the grid voltage is notsinusoidal and the inverter is targeting a sinusoidal output.

The differences in behavior between a closed-loop inverter control andan amplitude-corrected inverter control may not be distinguishable to asimple root mean squared (RMS) measurement of the waveform, butclosed-loop inverters with high switching frequencies may be capable ofrejecting harmonics above the 100^(th) order. While this behavior ispreferred to minimize output total harmonic distortion (THD), it mayalso result in large harmonic currents flowing between the closed-loopinverter and the power grid, connected synchronous machines, powerelectronics such as variable-frequency drives, or evenamplitude-corrected inverters, which may produce higher harmoniccontent, especially at higher frequencies.

FIG. 1 illustrates a time plot representing 5 seconds of responsecharacteristics to changing loads for three example DC energy sourceswith capability to produce or absorb energy. The battery voltage has amomentary dip due to the required time to accelerate charge carriers inthe electrolyte when load is applied at time T1. The capacitor voltagedoes not experience a dip at time T1, but begins slowly discharging attime T1. The fuel cell voltage dips significantly at time T1 until theinternal rate of ion transfer through the membrane has time to change.

By time T2, the capacitor voltage has decreased to a stable point,matching the input and output currents. By time T3, the fuel cell hasrecovered to a new nominal voltage level. The fuel cell voltage may notrecover to the open-circuit terminal voltage of the cell.

FIG. 2 illustrates a short time period of the behavior of the threesources illustrated in FIG. 1. The shorter time span allows review ofthe battery behavior when a load is suddenly applied. At time T4, theload changes and the battery voltage dips momentarily due to therequired change in the current to maintain output voltage. By time T5,the battery current has increased to the new required level, allowingthe voltage to recover. Battery voltage may not fully recover to nominalvoltage due to internal resistance and electrical potential required tomaintain charge mobility.

FIG. 3 illustrates an example supply current waveform from a battery toa grid-tie inverter sourcing single-phase 60 Hz AC power at unity powerfactor. The internal DC bus voltage displayed is applied to the DC linkinside the inverter, such as the DC link capacitor illustrated as 75 inFIGS. 7 and 81 in FIG. 8. The input current displayed is measured at theinput to the inverter from a 225V DC source.

The 120 Hz AC component on the DC current input to the inverterillustrated in FIG. 3 is due to the sinusoidal nature of the output.Both the positive and negative half-cycle source power to the load, butno power is sourced to a resistive load when the voltage crosses 0because the power is the instantaneous product of the voltage andcurrent. For a resistive load, the input power profile looks like a 120Hz sinusoid with a DC offset that would allow calculation of averagepower by multiplying by the input voltage.

The thickness of the current waveform illustrated in FIG. 3 is due toripple current on the inductor for the boost converter. The currentincreases while a boost switch is active, then decreases when the boostswitch is deactivated, increasing in voltage to allow the current tocontinue to flow in the inductor and allowing increase in voltage fromthe input voltage to the DC bus voltage. This ripple may have anundesirable effect on the source, depending on the size and type of thesource and the frequency of the boost converter switching.

FIG. 4 illustrates another example supply current waveform from abattery to a bi-directional grid-tie inverter sourcing single-phase 60Hz AC power at 0.8 power factor. In this case, the current waveformcrosses through zero, meaning that the inverter is charging the batteryfor portions of the output cycle. The energy to charge the battery istaken from stored energy in the inductive load element connected to theoutput of the inverter during portions of the alternating cycle when theenergy in the inductive load element is decreasing.

Although some sources, such as a battery or fuel cell, are capable ofabsorbing power, other sources, such as a solar panel or an AC devicesupplying DC power through a rectifier using simple diodes, are notcapable of absorbing power, even if these sources can handle ripple onthe current. While such devices may be capable of operating with a unitypower factor, where the current direction is always into the inverter,the devices are not capable of absorbing the current returned to thesource during the discharging of the energy storage element connected tothe inverter.

When attempting to return current to the source, the bi-directionalboost converter may apply the full voltage of the DC link within theinverter to the input. Some devices may be able to handle full busvoltage applied to their output. In this case, the DC bus voltage willincrease with the returning energy from the load. Some devices may notbe able to handle to full voltage of the DC bus in the inverter,especially in applications where the boost ratio of the converter ishigh (e.g. 1:10 boost). In these cases, the boost converter must preventreversal of the input current to prevent damage to the source.

Some boost converters utilize a diode for the connection between the DClink and the boost inductor or transformer. These boost converters areuni-directional converters, being unable to return current to the DCsupply. By design, it is not necessary for a uni-directional inverter tohave a uni-directional boost converter. For example, use of afield-effect transistor (FET) instead of a simple diode is a commonsolution to improve efficiency of an inverter, but allows bi-directionalpower transfer to and from the DC bus. Failure of the source due toovervoltage is not typically a concern with a uni-directional boostconverter but can still be a concern with a uni-directional inverter ifthe boost converter is bi-directional.

The DC average of the input current illustrated in FIG. 4 may still beused to compute the average power provided from the battery to theinverter by multiplying by the battery voltage. The average power isprimarily related to the resistive load on the inverter but doesincrease slightly with reactive load due to the alternating current thatis flowing through the boost converter.

Similarly to the input current waveform for FIG. 3, the input currentwaveform for FIG. 4 illustrates a significant thickness representing thehigh-frequency ripple on the waveform. This ripple is caused byreversing the voltage across the inductor while boosting voltage andoccurs at the switching frequency of the boost converter.

Referring to FIG. 2 to determine capabilities of the battery source forthe inverter illustrated in FIG. 4, the battery is capable of handlingthe low-frequency ripple at 120 Hz, but not the high-frequency ripple atthe switching frequency of the boost converter (e.g. 20 kHz to 200 kHz).A capacitor may be placed in parallel with the battery in order tosupply the rapidly-changing current demand. An inductor-capacitor (LC)filter, such as shown in FIG. 7, may also reduce the ripple current seenby the battery, with potentially enough decrease in capacitance toreduce the overall filter cost.

FIG. 5 illustrates the bus voltage and input current profiles for twoexample techniques of controlling a bi-directional boost converter.Referring for comparison back to FIG. 4 that illustrates a conventionalbi-directional boost converter control strategy, regulating the DC busvoltage to a substantially constant value, FIG. 5 illustrates analternate control strategy for a bi-directional boost converter controlstrategy where the input current is regulated to a substantiallyconstant value, while the voltage on the DC link is allowed to vary.This approach effectively utilizes the DC link capacitance as an energystorage element in lieu of using the power source as an energy storageelement or in cases where the power source is not able to store energy.A combination of the two approaches may be used.

FIG. 6 illustrates input current profiles for boost converters using twodifferent inductors. Referring for comparison back to FIG. 4 thatillustrates an input current profile for a boost converter using asmaller inductor, e.g. 260 uH, while FIG. 6 illustrates an input currentprofile for a boost converter using a larger inductor, e.g. 1 mH. Bothexample boost converters are switching at 100 kHz.

The width of the input current waveform illustrated in FIG. 6,representing the ripple on the input current, is significantly lowerthan the width of the input current waveform for FIG. 4. This decreasein ripple is a consequence of the increase of the inductance of theboost converter inductor. The higher inductance reduces the current slewrate, limiting the extent of the current change and therefore theripple.

While additional inductance for the boost converter producing thebehavior show in FIG. 6 would reduce the ripple on the input current, itmay not be immediately obvious that such a change may also decrease theresponse speed of the boost converter and increase the voltage rippleand settling time of the DC link capacitor 75 voltage. The largerinductance and corresponding decrease in the current slew rate increasesthe time required for the boost converter to change the input currentand thereby control the DC link capacitor 75 voltage. This increase involtage ripple and settling time may be considered an undesirableeffect, to be weighed against the undesirable effect of the inputcurrent ripple.

In addition, increasing the inductance of the boost inductor may resultin an increase in the cost of the boost inductor, a decrease in the peakcurrent capacity, or a combination of the factors. The physical size ofthe boost inductor may also increase, as well as the hysteresis andwinding loss in the inductor. Given the factors described above, it isdesirable to reduce the input current ripple without increasing theinductance of the boost converter inductor.

The examples illustrated above refer to a single-phase inverter case. Athree-phase inverter with balanced phase loading does not exhibit anyripple on the input current due to the relative phasing of the threephases. One phase is always drawing positive power in a three-phaseconfiguration, such that with a balanced load, the three current's sumto a DC input. Consideration of a single phase of a three-phase inverterallows support for a maximum imbalance on the output currents, which isa possible case in some applications.

Depending on the inverter source topology, inverters may not beconnected to a common source. Connection to separate sources, whileeffectively separating the inverters and requiring additional control ofthe boost converter, may provide additional benefits. Additionalbenefits may include allowing connection of the inverters to the gridwithout input or output isolation transformers, allowing support ofimbalanced loads, and reconnection into a variety of outputconfigurations. As an example, three isolated inverters may be connectedin series to provide increase voltage with a single-phase input. Asanother example, three isolated inverters may be connected in parallelto provide increase single-phase current. As yet another example, threeisolated inverters may be connected in a delta configuration.

FIG. 7 illustrates an example bi-directional boost converter containingan input filter in addition to the boost inductor 72. The input filterinductor 70 may have a significantly lower inductance than the boostinductor 72. As an example, the input filter inductor 70 may have aninductance of 1 uH while the boost inductor may have an inductance of260 uH. The input filter capacitor 71 may have a significantly lowercapacitance than the DC link capacitor 75. As an example, the inputfilter capacitor 71 may have a capacitance of 400 uF while the DC linkcapacitor may have a capacitance of 2000 uF.

The input filter inductance and capacitance requirements may bedetermined by a need to reduce the input current ripple to a givenlevel, by a need to provide a more stable input voltage to the boostconverter, a combination of both, or neither. The effective impedance ofthe input filter to the boost converter switching frequency (e.g. 20 kHzor 200 kHz) is significantly different than the effective impedance to acurrent variation frequency, such as that supplied to a single-phaseinverter (e.g. 100 Hz or 120 Hz).

Because the switching frequency directly effects the impedance of thefilter elements, an input filter designed only to remove the switchingfrequency ripple may not need to be large or expensive.

FIGS. 8A and 8B illustrates input current profiles for boost convertersutilizing an input filter in conjunction with different boost convertercontrol techniques. The input current profiles in the example stillcontain any low frequency variations in the signal (such as the currentvariation through the output cycling at 100 Hz or 120 Hz), but havegreatly reduced the high-frequency (e.g. 100 kHz) ripple on thewaveform.

In cases where the inverter is fed by a slow-responding device, such asa fuel cell, the input filter may be coupled with the alternativecontrol strategy illustrated in FIG. 8B. In cases where the inverter isfed by a quicker-responding device, such as a battery, the input filtermay be sufficient to allow the device to operate properly by minimizingthe high-frequency (e.g. 100 kHz) ripple on the battery input current,while the battery may handle the low-frequency (e.g. 120 Hz) ripplewithout trouble, as illustrated in FIG. 8A.

In some cases, it may be possible to detect the type of source feedingthe inverter and to select the correct operational strategy to operatethe source within its capabilities. As an example, the response dataillustrated in FIGS. 1 and 2 may provide sufficient differentiation toallow the inverter to determine the type of source feeding it.

FIG. 9 illustrates an example surge suppression device, operating tolimit the voltage on the DC link within a safe level. The surgesuppression device may only act under conditions where the inverteroutput receives a surge, such as when connected to the utility in theevent of a lightning strike. The surge suppression device may operate asingle time or multiple times. The number of times the surge suppressiondevice operates may be related to the intensity or amplitude of thesurge event.

The surge suppression device may contain an element or elements (e.g., avariable resistance component) designed to offer a variable resistanceor to conduct at a pre-defined voltage, such as a metal oxide varistor(MOV) 90 a, 90 b, gas discharge tube, thermistor, or Zener diode. Thevariable resistance component limits the bus or DC link to a voltage.The variable resistance component is coupled to the inverter directly,or indirectly by way of a fuse or other component. The surge suppressiondevice may include a semiconductor component which is activated bycontrol circuitry that provides a control signal to the semiconductorcomponent. The semiconductor component may be used in place of thevariable resistance component or in parallel to the variable resistancecomponent. The semiconductor component may be a crowbar that clamps tozero volts. This prevents the variable resistance device from beingdamaged or destroyed in a high voltage condition.

The control signal may specify a selected mode. As an example, the surgesuppression device may include an insulated-gate bipolar transistor(IGBT) which is operated in linear mode (e.g., selected mode) to clampthe bus voltage to a level. As another example, the surge suppressiondevice may include a latching semiconductor device, such as a siliconcontrolled rectifier (SCR), that drains the DC link voltage to apredetermined level or to 0 before resetting. In addition, the surgesuppression device may contain a non-resettable component, such as afuse 91 a or 91 b.

The surge suppression device may contain a plurality of variableresistance elements and/or a plurality of semiconductor elements. Thesurge suppression device and control circuit may contain failure sensinglogic 95 and failure sensing devices that allow for detection of thefailure of one or more elements in the variable resistance elementsand/or the semiconductor elements. The sensing devices may be sensingresistors 92 a, 92 b, allowing small currents to flow through variableresistance elements to prove that the elements still provide continuity.The sensing resistors 92 a, 92 b, are connected in parallel to thecorresponding variable resistance components (e.g., MOV 90 a, 90 b). Thesensing resistors 92 a, 92 b are example sensing components, and othercomponent types may be used for the sensing components.

The voltage across the sensing devices may be measured by voltage meters94 a, 94 b (e.g., voltage detection devices) to establish that theelements operate at the correct voltages. The voltage meters may provideinformation to the failure sensing logic 95. The sensing devices may beselectively connectable in the circuit using switching devices such asdry contacts of relays 93 a, 93 b. The relays 93 a, 93 b may becontrolled by the failure sensing logic 95. The relays 93 a, 93 b may beselectively activated by the failure sensing logic 95. The relays 93 a,93 b are connected to the sensing resistors 92 a, 92 b, respectively,and the inverter.

The surge suppression device may also contain a bus voltage sensor 94 c(e.g., direct current voltage sensor) that is coupled to the directcurrent link of the inverter. The bus voltage sensor 94 c may beconnected to the bus constantly or only be connected occasionally. TheDC link voltage sensor 94 c may be used to selectively activate asemiconductor element to decrease the DC link voltage to a predeterminedthreshold. The bus voltage sensor 94 c may provide information to thefailure sensing logic 95. The failure sensing logic 95 may useinformation from the DC link voltage sensor 94 c and the voltage meters94 a, 94 b to determine a conduction voltage for the variable resistanceelements, such as the MOVs 90 a, 90 b. The conduction voltage may becompared to a threshold (e.g. 400V) to determine if the variableresistance elements are operating as expected. The control circuitincluding the failure sensing logic 95 is configured to detect a failurein the variable resistance component or in the semiconductor componentin response to data from the direct current link voltage sensor.Specifically, the failure sensing logic 95 analyzes information from thevoltage detection device 94 a, voltage detection device 94 b, and thedirect current link voltage sensor 94 c to determine a conductionvoltage for the first variable resistance component and the secondvariable resistance component.

In the example illustrated in FIG. 9, the surge suppression devicecontains two variable resistance elements MOVs 90 a, 90 b. In thecondition where the conduction voltage of one of the series-connectedelements was lower than a normal operating voltage of the DC linkcapacitor 101 voltage, the conduction voltage of the MOVs 90 a and 90 bmay be measured by selectively activating relays 93 a and 93 b. Thefailure sensing logic 95 may compute the conduction voltage of MOV 90 aby closing contacts on relay 93 b and subtracting the voltage from thevoltage meter 94 b from the voltage from the voltage meter 94 c. Asimilar technique may be used to compute the conduction voltage of MOV90 b using relay 93 a and voltage meter 94 a.

If the normal operating voltage of the inverter is less than half themaximum DC link voltage, it may be necessary to extend the surgesuppression module to three or more variable resistance elements inorder to retain ability to test each individual element. This requiresmodification to the surge suppression module illustrated in FIG. 9, butthis modification merely involves addition of elements in series andselection of conduction thresholds such that the total seriescombination provides appropriate voltage protection. Addition of morevoltage meters and sensing resistors may be included.

A variable resistance element conduction voltage significantly over thethreshold (e.g., open circuit threshold) may indicate an open variableresistance element and result in the failure detection logic 95 settingfailure indication 96 to active. Similarly, a variable resistanceconduction voltage within a certain range (e.g., shorted range) of 0V(e.g. 5V) may indicate a shorted variable resistance element and resultin the failure detection logic 95 setting failure indication 96 toactive. The fuses 91 a, 91 b may provide a device to disconnect a failedsurge suppression device from the DC link, allowing the inverter tocontinue to operate.

If fuse 91 a or 91 b is blown, the failure sensing logic 95 may receivea reading of 0V from the DC link voltage sensor 94 c. A blown fuse tothe surge suppression module may indicate that it is no longer able toprotect against surges. This may result in failure indication 96 beingset active. The failure detection logic 95 may be fed by a supply 97,separate from the DC link, in order to enable fault indication with ablown fuse connecting to the DC link.

The failure indication 96 may be a digital signal to a controller orprocessor on the inverter, a visual indication to an operator, orinformation conveyed over a communication bus. The failure condition 96may be transmitted as a normally active signal, such as a digital signalthat is de-asserted when a failure is indicated or an LED that isnormally active (e.g. a green ‘good’ signal to notify the operator). Thefailure indication 96 may be a combination of these techniques oranother similar technique. The failure indication 96 may include amessage to another device. The message may be transmitted to an invertercontroller. The message may instruct another inverter to be broughtonline. The message may be a notification for a user to replace thesurge suppression device.

FIG. 10 illustrates an example inverter configuration containingmultiple redundant surge suppression devices 100 a, 100 b and outputovercurrent protection 108. Although two surge suppression devices 100a, 100 b are shown, additional or fewer surge suppression devices may beincluded. Overcurrent protection device 108 may also be used toselectively connect to the utility. An additional device may be used toselectively connect to the utility instead of using the overcurrentprotection device 108.

The surge suppression devices are connected to a DC link, supported byDC link capacitor 101, which supplies DC voltage to H-bridge 102 in theexample inverter. The surge suppression devices 100 a, 100 b, providelimiting for the voltage on the DC link and the DC link capacitor 101.In the event of a large surge, the surge suppression devices limitingthe DC bus voltage may result in the tripping of the overcurrentprotection device 108 due to conduction through the body diodes of theswitches in the H-bridge 102.

In the event that a surge suppression device 100 a, 100 b fails, thefailure indication 96 is received by the inverter controller 105 and maybe transmitted to the user interface 106. The inverter controller 105may be the controller that operates the switches in the H-bridge, thecontroller that operates the boost converter, or another controller. Theinverter controller 105 is configured to operate a plurality of switchesto provide an alternating current to an output. All functions may beperformed by a single controller 105. The user interface 106 may be anindicator light (e.g. system status normal), multiple indicator lights(e.g. operating, warning, fault), a communications bus to externalequipment, a text-based user interface, or a graphical user interface oranother similar user interface.

The surge suppression modules 100 a and 100 b may be fed by separate,isolated power supplies from power supply module 107. The separate powersupplies may allow for detection of faults even with fuses 91 a, 91 bblown. The surge suppression modules 100 a and 100 b may be sized toabsorb surge energy based on the size of the filter inductor 103 and thefilter capacitor 104.

Because surge suppressors must absorb large energy spikes, they oftenact sacrificially. When failing, they may fail to a short circuit or anopen circuit. In either case, the surge suppression modules 100 a, 100 bmay require replacement for continued reliable operation of theinverter. Providing multiple surge suppressors 100 a, 100 b and healthstatus for each allows for the inverter controller 105 to performprotection for other components, such as notifying an operator, in areplacement instruction message, when the first surge suppression module100 a fails so that it can be replaced, disconnecting the inverter fromthe grid using overcurrent protection device 108 or another deviceconfigured to disconnect the inverter from the grid. The invertercontroller 105 may also operate at a reduced output, provide an audiblealert, or perform another appropriate action when a surge suppressionmodule 100 a is no longer providing protection.

In cases where three or more surge suppression devices are provided, thebehavior may differ with one inoperable module versus two or threeinoperable modules. The behavior may differ for each inoperable module.As an alternative, the inverter operation may not change when allmodules are failed, but the inverter controller 105 may retain theinformation as to the failure timing for warranty considerations inevent of a failure due to a surge on the utility. Surge suppressionmodules 100 a, 100 b may be designed to permit replacement whileinverter is in operation (without taking out of service).

FIG. 11 illustrates a droop characteristic curve for an exampletransformer. The droop occurs due to the internal impedance of thetransformer. The transformer illustrated in FIG. 11 provides animpedance of 3%, giving an output decrease of 3% at full reactive load.Although transformer voltage dip with full real load is typically lowerthan the rated impedance, transformer voltage still drops appreciablywith real load as well.

FIG. 12 illustrates an example system containing a three-phase inverter120 a, 120 b, 120 c connected to a transformer 121 fed by the utility.The Inverters are connected between the three individual phases of thethree-phase service and the neutral point, provided by the transformer121. This connection topology may provide a lower DC link voltage (e.g.DC link capacitor 101 voltage) inside the inverter, potentially allowingfor lower cost capacitors and switches.

The inverter is connected to the output (secondary) of the transformer121, providing impedance between the inverter and the power grid. Inaddition, the underground feed lines 122, the bayonet fuses 123 and theoverhead lines 124 also provide impedance between the electricaldistribution grid and the inverter. The impedance presented by thecomponents on the high side of the transformer 121 may be reduced due tothe turn ratio within the transformer, but still provide some impedanceto the power grid.

In addition to the electrical distribution components illustrated inFIG. 12, the utility voltage may vary due to load on substationtransformers and tap configuration on multi-tap autotransformersthroughout the distribution system. The changes in the system voltagebased on load may not be restricted to variation from the distributiontransformer 121.

The elevated positioning and pervasive path of the overhead lines 124may make them susceptible to lightning strikes, causing surges throughthe electrical system. The surges are attenuated by the transformer 121,but still couple to the transformer secondary either through internalcapacitance or through differences in the ground reference of thesecondary and primary. This is often caused by large current flowingthrough the ground from the lightning strike. When coupled to thesecondary of a utility distribution transformer 121, inverters may notsurvive long without surge suppression.

Due to the accuracy of the output regulation on an output-correctedinverter, the inverter may not share load with the utility equally. Ifthe load changes, the inverter may adjust to provide appropriate outputvoltage, but the utility transformer sources current based on the droopcurve, so it may not source any additional reactive power if the outputvoltage remains constant. As a result, the inverter may support any andall load transients in order to provide voltage stability.

FIG. 13 illustrates an example current amplitude flowing from theinverter 120 and transformer 121 of the system in FIG. 12 with achanging load connected to the secondary of the transformer, which isalso connected to the output of the inverter.

The load changes from 0 kVA to 105.9 kVA at time T6, resulting in ashort-term dip in the output voltage of approximately 0.8V and somepower being taken by the utility. The amplitude-correction in theinverter acts to correct the output voltage to 277V, reaching the targetat time T7, at which time, the utility is no longer supplying power. Attime T8, the load decreases to 26.6 kVA resulting in a short-termincrease in the load voltage and negative power supplied by the utility(the utility is absorbing power). The amplitude-correction in theinverter acts to correct the output voltage to 277V, reaching the targetat time T9 and the utility power reverts back to 0.

FIGS. 14A and 14B illustrate two example inverters. FIG. 14A is a blockdiagram representing an inverter with output amplitude correction. FIG.14B is a block diagram illustrating an inverter with closed-loop outputfeedback. An inverter with neither is simply the output-correctedinverter without the amplitude feedback 143 or root mean squared (RMS)calculation 144.

The output-corrected inverter of FIG. 14A provides a sinusoidal pulsetrain from a pulse train generator 140. The pulse train may be generatedbased on any or all of the following characteristics: Amplitude,Frequency, and Phase. If phase is used in the generation of the pulsetrain, the phase may be measure relative to a dominate phase of theinverter or may be generated relative to a fixed reference.

The amplitude of the sinusoidal pulse train may be adjusted based on theRMS amplitude of the output waveform, computed in the RMS calculation144. As an atypical alternative, the output may be adjusted based on anaveraged quantity or even a fundamental component from an FFT applied tothe output. The pulse train is provided to the h-bridge 141 to controlthe duty cycle provided to the switches in order to drive a sinusoidaloutput current. The output-corrected inverter in FIG. 14A may alsoinclude a feed forward or oscillation damper term for the output filter.This may be accomplished by a phase shifted current correction factor oran intentional distortion applied to the pulse train before reaching theh-bridge 141.

The switched voltage output from the h-bridge 141 is fed to the outputfilter 142, which removes most of the switching noise from the signal.As an advantage, the output filter 142 also provides an output impedancethat can be very helpful to prevent switch damage in a short-circuitcondition. The output filter 142 may be sized to allow sufficientimpedance to prevent excessive output current when fed a sinusoidalinput waveform and short-circuited on the output. The pulse traingeneration will typically fold back in a short current condition (whenthe output voltage measures very close to 0), but may require a fewcycles to do so to maintain stability.

An inverter without output amplitude correction may not provide thecapability to fold back in a short circuit. In addition, an inverterwithout output amplitude correction may have a stiffer filter to reducethe inherent droop associated with the filter. In some cases, inverterswithout output amplitude correction are unable to self-protect in theevent of a short circuit on the output. In other cases, theself-protection is accomplished by other devices, such as a fuse orcircuit breaker.

The closed-loop inverter of FIG. 14B generates a target, such as a sinewave, in the target generation 145 block. The target may be generatedbased on one or more of an amplitude, a frequency and a phase. Thetarget may be a sum of sine waves of different frequencies, the targetmay be derived from a lookup table. The phase may be relative to anothertarget signal or to a fixed reference. The controller 146 measures thesignal feedback 147 and may compare the signal feedback 147 with thetarget 145. The signal feedback 147 may contain output voltage, outputcurrent or both signals. The controller 146 may control voltage orcurrent to the target and may involve both signals in the controlcalculations. For example, a current controller may measure the outputvoltage in order to establish the necessary voltage to provide at theinput to the filter inductor in order to achieve the target current inthe next cycle.

The controller 146 may be a proportional-integral-derivative (PID)controller, a more complex linear controller, a state-space controller,a model optimization controller, or a combination of these or othercontrollers. The controller 146 may switch control modes based onobserved behavior, such as current exceeding a threshold or outputvoltage suddenly dropping. The controller 146 may compute additionalfactors for consideration, such as output impedance, when computingoptimal switch responses.

The signal feedback 147 may be measured once per switching cycle of theinverter, multiple (e.g. 16) times per switching cycle of the inverter,or even less frequently than once per switching cycle of the inverter.In many cases, system stability improves with increasing sample rates,making systems with less frequent sampling than once per switching cycleuncommon. Increased switching frequency will often improve stability ofthe inverter 120 with the added benefit of improving the range ofharmonics that the inverter can reject.

The controller 146 may include a model of the filter 142. The controllermay dynamically determine the characteristics of the filter. The dynamicdetermination of the characteristics may be limited to a range or amaximum difference from an expected value. The controller 146 maymeasure internal values within the filter 142. The controller 146 mayestimate internal values within the filter 142. The controller 146 mayestimate a future state of the measured or estimated values within thefilter 142. The controller may limit duty cycle based on a predictedfuture state of the current through the filter inductor 103 in thefilter 142.

Different inverter topologies may provide different features, behaviorand capability. For example, an inverter without amplitude-correctionmay not provide a consistent voltage droop curve, depending onvariations in the output filter, DC link stiffness and switch impedance,all of which may vary with temperature. And amplitude-corrected invertercan provide a consistent droop characteristic but may not be able tocorrect harmonic distortion in the output signal. An output-correctedinverter may be able to operate with a higher-impedance output filterthan a non-output-corrected inverter, allowing short-circuit withstandcapability. A closed-loop inverter may be able to operate at asignificantly lower filter impedance and still provide short-circuitwithstand capability.

In addition to the behavior illustrated by FIG. 13, the utility voltagemay experience variation in the primary side of the utility transformer,changing the no load point on the transformer droop curve. In this case,the inverter output voltage may also vary when the inverter 120 iscoupled directly to the secondary of the transformer 121. Severaltechniques exist to accommodate this variation in utility voltage,including controlling the inverter current relative to the measuredoutput voltage, disabling operation when no utility is sensed, andoperating with a very steep droop curve.

One of the advantages to providing current based on measured utilityvoltage is that the power factor that the inverter outputs can easily bedefined by the relative phase between the measured utility voltage andthe current sourced by the inverter. Another advantage is that thecontrol is simple and straightforward.

One disadvantage to providing current based on measured utility voltageis that the inverter is unable to operate without utility voltagepresent. This creates a concern for the inverter system, as inverter areoften used with battery banks to provide reliable ride-through poweruntil an alternate power source, such as a generator, can be broughtonline.

Operating on a very steep droop curve may provide the advantage ofstable operation when paralleled with utility and ability to operatewithout utility present. The voltage droop curve changes the reactivepower output of the inverter 120, allowing the inverter to continue toprovide real power without concern. Droop systems often provide afrequency droop characteristic with real load, but this may not benecessary when operating in parallel with the grid. If provided, thefrequency droop may cause the output power to fluctuate, which may notbe desirable behavior for the inverter if it is supposed to provide afixed output power or if the output power needs to match the input powerfrom a system that is not able to respond quickly.

With simple reactive droop, the inverter may still be able to fix theoutput power in order to load the sources supplying the inverter 120optimally while controlling voltage due to reactive droop. If thevoltage starts to drop, the inverter 120 will increase reactive poweroutput, reducing the reactive load on the transformer and stabilizingthe transformer 121 secondary voltage.

One drawback to reactive droop is that the inverter may have to providesignificantly more apparent power than would otherwise be required tooperate. In other words, the inverter may operate at a low power factor.Because the current passing through the H-bridge 102 of the inverter islinked to the output apparent power, not the output real power, theinverter efficiency may decrease significantly when operating at a powerfactor other than unity.

Another potential drawback to operating in droop is that the inverter120 voltage is linked to the output reactive power, meaning that theoutput voltage of the inverter 120 may vary dynamically when the loadchanges. As an example, a large motor starting may cause significantlight dimming throughout the facility, even though the inverter 120 maybe fully capable of starting that motor with no noticeable voltage dip.

As an alternative to current-controlling and reactive droop, theinverter 120 may be equipped with a variety of operating modes and logicto switch between them. For example, the inverter 120 may operate in agrid-tied mode where the output current was controlled relative to themeasured waveform when connected to the grid, then switch to a fixedoutput voltage and frequency when no longer connected to the grid.

While selecting modes may seem to be the best strategy for operating agrid-tie inverter, it brings with it additional challenges andadvantages. One advantage is that the inverter 120 output is as stiff asthe inverter topology will allow while operating in islanded mode, whichmay be orders of magnitude better than utility voltage regulation ifequipped with amplitude-correction or operating in closed-loop. Anotheradvantage is that the inverter 120 may be capable of operating at avariety of power factors, including unity, allowing the inverter tooperate at maximum efficiency.

One potential disadvantage to selecting modes for the inverter is thepossible need to operate with different behavior in a given situation.For example, it may be desirable to maintain consistent voltage at acertain location, while another location may prefer optimal inverteroperating efficiency.

FIG. 15 illustrates an example system containing inverters 150 a-cconnected to a synchronous electrical machine 151 driven by a primemover 152. The three-phase inverter is connected in a wye configuration,with each inverter element 150 a, 150 b, 150 c connected between theoutput phase and the neutral. This topology matches the connection ofthe internal coils in the synchronous electrical machine 151.

When operating in parallel with a synchronous electrical machine 151driven by a prime mover 152, load sharing may become important. In somecases, the inverter may provide only the power which is supplied by theenergy source feeding it, but in other cases, the inverter may havelarge energy reserves to draw from and replenish, meaning that theinverter may be put to different uses.

When in parallel with a synchronous machine, sudden sourcing of reactivepower will result in the load voltage increasing. Similarly, suddensourcing of real power will cause the load frequency to increase whenthe synchronous machine is driven by a prime mover 152. The prime mover152 will typically act to correct output frequency to a nominal value,but the control response may not be a quick as that of the inverter. Thesynchronous electrical machine 151 will typically react to correctoutput voltage to a nominal value, but the control response may not beas fast as that of the inverter.

In some cases, the inverter response is one or more orders of magnitudebetter than that of the synchronous electrical machine 151 or primemover 152. As an example, the inverter may be able to increase from nooutput power to full output power in 400 us, while the prime mover 152may require 4 seconds to increase fuel and airflow from no power to fullpower and the synchronous electrical machine may require 400milliseconds to increase excitation from no-load to full reactive power.Due to this difference in performance, the inverter is often used torespond quickly to a transient condition, allowing the synchronouselectrical machine 151 and prime mover 152 time to respond. Thiscondition may result in a temporary overload for the inverter, butoverheating may not be a concern for a brief period.

An inverter may need to operate with different behaviors depending onwhat the output is connected to. In addition, different invertertopologies may provide different features and capabilities. In additionto the abilities already discussed, closed-loop inverters may provideimproved power quality by actively sourcing harmonic current to offsetharmonic distortion in a waveform from another source. If the target isa generated sinusoid, a closed-loop inverter may be able to generate asignal with 0.1% THD or less, even when feeding an inductive orcapacitive load. In addition, a closed-loop inverter with sufficientlyhigh switching frequency may be able to offset distortion from variousnon-linear loads operating at significantly lower switching frequencies.For example, an inverter switching at 100 kHz may be able to providesignificant correction to harmonic distortion caused by a variablefrequency drive switching at 2.4 kHz.

Both the output from the utility transformer 121 and the synchronouselectrical machine 151 may contain harmonic content. This content may beobserved using a frequency-domain analysis of the signal or by visualinspection of the shape of the output waveform. In most cases, thisoutput is sufficiently clean to operate most electrical equipment, butin some cases, particularly where non-linear loads are involved, theoutput voltage becomes distorted outside a limit where devices receivingpower from the source exhibit abnormal behavior.

Due to the frequency of the harmonic content all falling above thefundamental frequency, the energy content to remove from the signal isminimal, meaning that many inverters may be able to store the necessaryenergy to correct harmonic distortion in their DC link capacitors. Withno additional hardware required, the desire to use an inverter as avoltage filter is often sufficient to lead to use in that application.

Even in cases where the switching frequency of the non-linear loadexceeds that of the inverter, the capacitance 104 of the output filteron the inverter will still provide an additional source forhigh-frequency current. Ordinarily, capacitance tends to drivesynchronous electrical machines and transformers, to a lesser degree,unstable, but the inverter is able to control current to the capacitorto some respect and may be able to make the capacitor appear to be anytype of load desired (including an inductor) with the right topology.

FIG. 16 illustrates example current waveforms of current flowing betweenthe inverter and transformer of the system in FIG. 12 and is similar tobehavior exhibited by the inverter and synchronous machine of the systemin FIG. 15 when the inverter is a closed-loop inverter. This harmoniccurrent is sourced by the inverter in order to cause a resultingharmonic current from the utility transformer 121 or synchronous machine151 that works with their internal impedance to correct the output ofthe synchronous electrical machine 151 or the transformer 121 to asinusoid or very close to a sinusoid.

For the example in FIG. 16, the total harmonic distortion (THD) of theutility and generator was 3.2%, but the harmonic current sourced by theinverter was able to reduce the THD for the combined output to 0.4%. Thepower sourced by the inverter is not significantly higher than if theinverter were not removing the harmonic content on the waveform, butthis is due to high relative impedance of the transformer or synchronousmachine.

As an example, the inverter current is well within its capability if a15 kW inverter is paired with a 60 kW generator set or a 25 kVA utilitytransformer. In the same system, a 100 kW inverter would barely beimpacted by the harmonic current and would probably be able to correctthe THD to 0.2% or even better. That same 100 kW inverter paired with a4 MW generator or a 1.5 MVA transformer would see significant harmoniccurrent, probably close to the inverter's current capability, and onlybe able to correct the THD to 1.1%.

While correction of harmonic distortion may be useful in some cases, inother cases, such as the example of the 100 kW inverter and 4 MWgenerator or 1.5 MVA transformer, it requires the inverter to sourcehuge currents with very minimal sourcing of net power. This results inthe inverter having a very low power factor and leads to heating withminimal to no benefit, reducing the efficiency of the inverter andpotentially reducing the life of internal components. In such cases, itmay be beneficial to adjust the internally generated target to match theoutput voltage from the synchronous electrical machine 151 or utilitytransformer 121.

At some locations, more than one inverter is operated in parallel. Inthis case, the effective impedance of the inverters is computed as ifthe impedances were in parallel, decreasing with each additionalinverter. If the site with a single 100 kW inverter had 40 100 kWinverters in parallel, the harmonic current required to correct the THDof the system to 0.3% or so would barely be noticed by each inverter, asthey would only be carrying a 40^(th) part of the current. Once theadditional inverter capacity is added to the example site, the harmonicdistortion no longer presents a problem.

Unfortunately, multiple inverters in parallel presents another problem.If there are very stiff sources controlling to a pre-generated target,what prevents them from conflicting as to the desired output andexchanging current between them instead of providing it to the load.

FIG. 17 illustrates an example system containing a plurality ofinverters with closed-loop output feedback connected together to acommon load. Each inverter is capable of sourcing of sinking currentfrom the common bus and the total current must equal the current to theload, but it is possible for inverter 170 (e.g., phases 170 a, 170 b,and 170 c) to source 200A, inverter 171 (e.g., phases 171 a, 171 b, and171 c) to absorb 200A, inverter 172 (e.g., phases 172 a, 172 b, and 172c) to source 35A and inverter 173 (e.g., phases 173 a, 173 b, and 173 c)to absorb 35A, with the load 174 receiving no current. All inverterswould be generating heat from the current flowing through their switchesand output filters, but not producing any work. In this case, voltageand frequency droop may help the inverters 170, 171, 172, 173 to reach acommon target and reduce their circulating current, but it may taketime. A different strategy to equalize the inverters would be to havethem communicate with each other, but baring close enough physicalproximity to allow communication, the inverters must have a techniquethat allows correction based on behavior alone.

Correcting output to match another inverter may require adjustment offrequency until the output signal lie in phase, which is how the drooptechnique would eventually correct them, but an alternate techniquewould be to stop trying to source current to drive the bus to a targetand instead wait for a single inverter to establish a signal that theothers can match once established. While this requires a protocol orprocedure to determine the inverter that is permitted to establish thevoltage, it allows for very quick arbitration of output and nearlyimmediate synchronization (possibly within a half cycle or less).

Once the signals match, a tiny increase in frequency when current lagsvoltage or a tiny decrease in frequency when current leads voltage willresult in correcting this current with minimal effect to the load andallow continued parallel operation. This technique may providesatisfactory performance in other applications, such as parallel toutility cases as illustrated in FIG. 12 or parallel operation with asynchronous electrical machine as illustrated in FIG. 15.

While behavior may be defined for each of the operational modes of theinverter 120, another potential disadvantage to selecting modes for aninverter is the need to sense the application in order to select anappropriate mode. This may be performed in a variety of ways, includingsensing on connectivity status of interconnects or selection ofoperational modes based on digital input to the inverter. A preferredmethod for selecting a mode would be based on characterization of theload being supplied.

For example, parallel operation with other inverters may be detected byvery large reactive current, possibly reaching a current limit, withlittle to no real current. This may be corrected by adjusting frequencyif the difference in phase is minimal, but may require disabling theoutput and re-synchronizing if significantly out of phase (as indicatedby reaching the current limit with voltage on the system).

Other ways to detect parallel operation with other inverters includesensing high-frequency ripple, measurement of effective impedance,response to a disturbance, impedance to a known potential, or anothertechnique of differentiating inverter behavior from utility ofsynchronous machine behavior. Different inverter topologies may havedifferent behavioral characteristics. Some techniques may be moreeffective for some inverter topologies than for others.

Differentiation between parallel operation with a utility source andparallel operation with a synchronous electrical machine may includeobservation of frequency behavior with large load changes as compared tovoltage behavior. As an example, the voltage may exhibit behaviorsimilar to transformer droop when the inverter output power changes,such as increasing voltage when the inverter output power increases,when the inverter is connected to the utility source. By comparison, thesynchronous machine voltage may not change appreciably with increasingoutput power form the inverter, but the frequency increase when theinverter power suddenly increases.

Detection of the type of paralleled source to an inverter may involveslow changes, such as adjustment over the course of 100 seconds, normalchanges, such as adjustment over the course of a second, and quickchanges, such as a change in 10 ms. Different sources may exhibitdifferent behavior to different types of changes. For example, asynchronous machine driven by a prime mover may control frequency andvoltage to a relatively constant value when subjected to a slow change,while utility voltage may vary.

As another example, a synchronous machine driven by a prime mover maycontrol voltage to a relatively consistent value while the inverterincreases power through a normal change, but the prime mover may not beable to control frequency during the event. A utility transformer maymaintain frequency within a relatively tight band with a normal changein the inverter output power, but the voltage may change withoutrecovering.

As yet another example, a synchronous electrical machine driven by aprime mover may see increases in voltage and frequency for a quickchange in the output power of the inverter. The voltage and frequencyrecovery time may follow expected intervals, for example, the voltagemay recover within 200 ms and the frequency may recover within 1 s.

Some synchronous electrical machines may have a droop characteristiccurve related to the output reactive power from the machine. Suchsynchronous electrical machines may exhibit different behavior from autility transformer in that all droop observed occurs due to reactivepower, while some utility droop may correlate with real power.

Another technique to determine if an inverter is connected to asynchronous electrical machine or a utility transformer relies on thedifferences in the electrical characteristics between the two systems.While a synchronous electrical machine may contain an air gap betweenthe rotor and stator to allow relative rotation, a utility transformermay not contain such an air gap. An air gap may change the magneticproperties of the flux path within the machine, such as thefrequency-based saturation or hysteresis behavior. Such behavior may berecognizable by a differences in impedance change with changingfrequency.

To detect different impedance behavior, the inverter may introduce ahigher-frequency (e.g. 5 kHz) carrier frequency on the output voltage.The inverter may measure the incoming waveform with no output current,and observe the harmonic current required to correct the THD to a knownlevel. The corrected THD level may be converted to the frequency domainusing a transform, such as a Fourier transform or fast Fouriertransform.

The inverter may learn characteristics of the different sources based onmeasurements taken by the inverter and a feedback to determine theactual source. The inverter may make use of a machine learningalgorithm, some combination of the techniques explained above, oranother technique for recognizing the sources based on known behavioralcharacteristic of the sources or general expected source behavioralcharacteristic.

FIG. 18 illustrates closed-loop inverter behavior into a short circuitcondition. The voltage remains very close to 0 and the output currentquickly reaches the current limit of the output. The controller 146limits the duty cycle of the PWM signal to the h-bridge in order toprevent current exceeding the current limit setting for the controller146. In this condition, the output voltage is very close to 0, allowingthe inverter to detect a short-circuit condition. This condition may beharmful to the output filter or switches in the h-bridge, although theloading is minimal on the boost converter or source, so it may benecessary to limit the duration of this condition. If the duration islimited, the inverter may fold back to a lower current or disable theoutput altogether.

If the inverter has disabled the output due to a short circuitcondition, it may be helpful to test the output periodically todetermine if the short circuit still exists. In another consideration,it may be beneficial to remain disabled after the condition is sensed.

In other applications the inverter may be connected without a groundreference or connected to a ground reference internally to the invertersuch that a fault to ground may be detected. In this case, the invertermay disable the output immediately, actively controlling to 0V anddisabling the output. Due to the nature of the output signal on theinverter, the waveform must fall from the crest of the sinusoid to thenext zero crossing within ¼ cycle, meaning that the output could becompletely disabled within about 4 ms on a 60 Hz output or 5 ms on a 50Hz output. This may provide improved ground fault detection relative toa standard GFCI breaker or standard GFCI outlet.

It may also be advantageous to control THD or to stabilize frequencywhen the site is fed by a generator. In addition, load sharing with agenerator is different than load sharing with the utility due tofrequency fluctuation, yet both are parallel operation modes.

The applications and functional control modes for an inverter outlinedabove are provided for exemplary reference and are not intended to limitthe operational or functional control modes of an inverterunnecessarily. Other control techniques may be used, such as voltagestabilization, frequency stabilization, and peak shaving, both tominimize peak demand from a utility source or generator source and tosource power to the grid when the time-of-day rate for the power is at amaximum. These modes and other similar modes may be selected based ondetermination of a state of the system or parallel operation with aspecific source.

In addition to differentiation of sources operating in parallel with aninverter, it may also be desirable to detect changes in thecharacteristics of a given source and to act accordingly. For example, autility energy rate change may cause system behavioral changes, as wellas changes in the utility feed line, transformer, substation, or similarcharacteristics, potentially indicating a back-feed condition (fed by anabnormal source).

Other factors may also lead to changes in the operational mode orbehavior of the inverter or inverter system. For example, increasedatmospheric temperature may require the inverters to operate at a lowerpower output. While some factors may be internal to the inverter system,others may be external to the system.

FIG. 19 illustrates an example controller 24. The controller 24 mayinclude a processor 300, a memory 352, and a communication interface303. The controller 24 may be connected to a workstation or anotherexternal device. The controller 24 may be connected to an input device304 such as a control panel and/or a database for receiving user inputs,system characteristics, and any of the values described herein. Thecommunication interface 303 may be connected to a sensor array 311including any one or combination of the sensors and receive sensormeasurements as described above including voltage meters 94 a, 94 b andbus voltage sensor 94 c. The communication interface 303 may beconnected to a driver array 312 for driving and controlling any of theswitches in the inverters. Additional, different, or fewer componentsmay be included. The processor 300 is configured to perform instructionsstored in memory 352 for executing the algorithms described herein.

The controller 24 may be connected to an output device 314, which may beintegrated with the input device 304 as the control panel and maydisplay settings or measurements of the controller 24. The output device314 may include a failure indicator for indicating the failuredetermined by the failure sensing logic 95. The output device 314 mayinclude an LED or other light or illuminator.

FIG. 20 illustrates a flow chart for the operation of the controller 24.Additional, different of fewer acts may be included.

At act S101, the controller 24 receives first measurement data from adirect current link voltage sensor of the inverter. The direct currentlink voltage sensor may be integrated into the controller 24 or beprovided separately in the surge suppression device or the invertercircuit.

At act S103, the controller 24 receives second measurement data from avoltage detection device configured to measure a parameter (e.g.,voltage across) at the sensing resistor. There may be two sensingresistors. One sensing resister is associated with a positive node ofthe direct current link of the inverter, and the other sensing resistoris associated with a negative node of the direct current link of theinverter. The variable resistors are connected to the inverter by way ofa switching component.

At act S105, the controller 24 performs an analysis of the firstmeasurement data from the direct current link voltage sensor and/or thesecond measurement data for the voltage across the sensing resistor.

At act S107, the controller 24 determines a conduction voltage for thevariable resistance component based on the analysis. There may be twovariable resistance components coupled together at a central node. Oneof the variable resistance components is coupled to the positive node ofthe direct current link of the inverter and the other variableresistance component is coupled to the negative node of the directcurrent link of the inverter.

At act S109, the controller 24 performs a comparison of the conductionvoltage and a threshold. Based on the comparison, the controller 24detects a failure in the variable resistance component in response toinformation from the variable resistance component and information fromthe detection device for the parameter of the sensing resistor. In oneexample, the controller 24 compares the conduction voltage for the firstvariable resistance component to an open circuit threshold and determinean open variable resistance element or open condition when theconduction voltage exceeds the threshold. In one example, the controller24 compares the conduction voltage for the first variable resistancecomponent to a shorted range and determine shorted variable resistanceelement or open condition when the conduction voltage is within theshorted range.

At act S107, the controller 24 signals a failure for the inverter. Thesignaling may include an indicator illuminated in response to thecomparison. The signaling may include a communication message sent bythe communication interface 303 to another device. The communicationmessage may be sent to the inverter controller, which may be configuredto disconnect the inverter from a power grid in response to the failure.The communication message may be sent to a management device that allowsa user to monitor surge suppression modules across multiple systems anddispatch maintenance personnel to replace the modules as needed. Thecommunication message may be sent to a manufacturer device for loggingerrors that occur in the installed systems.

An inverter circuit may change direct current (DC) to alternatingcurrent (AC) or change AC current to a different AC current. Variouscircuit topologies are available to perform this function. One examplefor DC-AC is an H-bridge topology with a predetermined number ofswitches arranged in an H-shape, which is described in more detailbelow. The switches of the inverter circuit are controlled to produce aspecific output, which may be a sinusoidal function (e.g., sine wave) ata predetermined frequency for the AC signal. The inverter circuit drivesa load, including a load circuit, which may experience a ground fault. Aground fault is an electrical contact between an energized path and aground path in the load circuit. The electrical contact may be anaccident or an intentional grounding (e.g., for testing). The groundpath may be the chassis or frame of a piece of equipment including theload circuit. The ground path may be the earth or another path that isexternal to the piece of equipment. While the ground fault may be causedby a breakdown in insulation, often times an animal or human becomes theelectrical conductor between the energized path and the ground path thatcauses the ground fault.

FIG. 21 illustrates an example circuit 11 for ground fault minimization.The circuit 11 includes a power supply rail 20 (DC link), a plurality ofswitches including switches SW1, SW2, SW3, and SW4. The voltage of thepower supply rail is V_(DC) connected to a power supply (e.g., source orvoltage source). From a circuit topology perspective, the switches arearranged in a first side, including SW1 and SW3 and a second side,including SW2 and SW4. An inductor 22 is connected between a first sidepair of switches SW1 and SW3 and an inductor 23 is connected between asecond side pair of switches SW2 and SW4. An output of the circuit 11 isin parallel with capacitor 21 and connected to the inductor 22 and theinductor 23. The capacitor 21 or the capacitor 21 in combination withthe inductor 22 and/or the inductor 23 is an output filter. Additional,different or fewer components may be included. The following embodimentsmay be applied to a synchronous inverter or AC-AC inverter that changesAC from a source to AC at the output.

FIG. 21 illustrates a single phase system. Alternatively, a three-phasesystem may be used. A three-phase system may comprise three circuitssimilar to circuit 11 connected in a predetermined configuration, suchas a wye configuration or a delta configuration, or a configurationfeeding all phases from a single DC link, such as a 6-switch line-lineinverter topology. Similarly, a n-phase system (e.g., with n equal toany integer greater than 1) may include n instances of circuit 11. Anyof the embodiments described herein may be adapted for a n-phaseimplementation.

FIG. 22A illustrates a chart 31 for an example output 32 of an invertercircuit. A sinusoid (e.g., sine wave) is illustrated but other waveformsmay be used such as a square wave or saw tooth wave. The circuit may becontrolled such that a pair of switches (SW1, SW4) provides a positivevoltage and current to the output and another pair of switches (SW2,SW3) provides a negative voltage and current to the output. The circuitmay be controlled using pulse width modulation (PWM) such that a controlsends PWM signals to the switches. FIG. 22B illustrates an example PWMcontrol signal for the inverter of FIG. 22A. The pulse width of the PWMsignal may be proportional to the amplitude of the output of theswitches. That is, when the output filter smooths the PWM signal to asinusoidal output, the longer pulses of the PWM signal correspond to thelarger amplitudes of the sinusoidal output. A combined switch signalincludes a first pulse width modulated signal for the first side pair ofswitches and a second pulse width modulated signal for the second sidepair of switches.

FIG. 22B illustrates an example chart 33 for a PWM signal 34. Chart 31and chart 33 are aligned in time so that the increased width in thepulses of the PWM signal 34 correspond to the higher amplitudes in theoutput 32. The switches SW1, SW2, SW3, and

SW4 may be referred to collectively or individually as the switches.

FIG. 23 illustrates examples for the switch of FIG. 21. The switch maybe composed of a single device, such as the FET 25 a or the IGBT 25 b,or multiple devices in a common package, such as the combined FET andbody diode 25 c or the combined IGBT and body diode 25 d. The switch maybe composed of multiple devices connected in series, such as two FETs 25e or two IGBTs 25 f. Greater than two devices may be connected inseries, the series connection may include steering devices and energystorage or dissipative elements to equalize voltage across the switches.

The switch may be composed of multiple devices connected in parallelsimilarly to the parallel-connected FETs 25 g and the parallel-connectedIGBTs 25 h. The switch may be composed of a parallel combination of aFET and an IGBT 25I or a combination of one or more IGBTs and one ormore FETs connected in parallel. The switch may be composed by aback-to-back combinations of FETs 25 j or a back-to-back combination ofIGBTs 25 k. Both source and drain connected FETs and emitter andcollector connected IGBTs are considered as back-back combinations.

FETs included in 25 e-25 k may be simple FETs 25 a or combined FETs withbody diodes 25 c. IGBTs included in 25 e-25 k may be simple IGBTs 25 bor combined FETs with body diodes 25 d. In addition, the concept of aswitch is that of a means of interrupting or significantly reducingcurrent flowing through a path and is not restricted to the embodimentsdetailed herein.

FIG. 24A illustrates a chart 35 with output 32. At time T_(f) a groundfault occurred at the load. The ground fault is determined at theinverter, which is discussed in more detail below. The output 32 abefore the ground fault, illustrated as a solid line, follows thesinusoidal signal generated from the increasing/decreases widths of thepulses from the PWM signal. The uncontrolled output 32 b, illustrated bya dotted line, illustrates the output with no control in response to theground fault. The controlled output 32 c, after the ground faults occurillustrates that the inverter is actively controlled to a thresholdoutput such as a zero output in a short amount of time, ending at zerooutput point t_(z). The switch signal in response to the ground faultsignal changes a state of at least one of the switches and may changethe state of two, three, or four of the switches.

The maximum amount of time between the fault t_(f) and the zero outputpoint t_(z) when the output substantially reaches zero may be less thana predetermined amount. The predetermined amount of time for the maximumdelay between fault t_(f) and the zero output point t_(z) may be afraction of the period of the output 32. The predetermined amount oftime for the maximum delay between fault t_(f) and the zero output pointt_(z) may be a ¼L where L is the wavelength or period of the output 32.For example, for 60 Hz, the period of the cycle is 16.67 milliseconds,and ¼ of the period of the cycle is 4.2 milliseconds. Thus, for a 60 Hzsignal the maximum delay between the ground fault signal and thesubstantially zero output from the inverter is 4.2 milliseconds.

A controller is configured to receive a ground fault signal thatindicates a fault has occurred and configured to generate a switchsignal for the switches for a minimum energy state of the output filterand in response to the ground fault signal. The inverter may be activelycontrolled to the threshold (e.g., zero) by inserting one or more PWMpulses into the PWM signal provided to the switches of the inverter.FIG. 24B illustrates a reverse pulse 37 that is provided to the outputof the inverter in order to discharge the capacitor 21. The reversepulse 37, as illustrated is negative when the PWM signal 34 is positive.Conversely, the reverse pulse 37 is positive when the PWM signal 34 ispositive.

Under normal operation, the voltage of the capacitor 21 changes as thecapacitor 21 is charged and discharged by the PWM signal 34. As currentleaves the capacitor 21 the current flows to the load, and during aground fault, the current may flow to an unintended sink such as a humanor animal. However, by inserting the reverse pulse 37 to the output ofthe inverter, current from the capacitor 21 is reverse conducted throughinductor 22 or inductor 23 to the DC link 20. Specifically, the reversepulse 37 may adjust the voltage in inductor 22 and/or inductor 23 sothat a reverse current flows through inductor 22 and/or inductor 23 andback to the DC link 20. The amount of current that flows to the DC link20 is not available to flow to the unintended sink. If all of the storedenergy in the capacitor 21 flows to the DC link 20, no current will flowto the unintended sink.

Subsequent to controlling the output of the circuit to the thresholdoutput which is substantially zero voltage and/or zero current, thecontroller may send a switch signal to the switches of the circuit thatturns off or opens all of the switches. In other words, in response to aground fault detection, first the switches are controlled so that thecircuit output is substantially zero, and then the switches are turnedoff.

FIGS. 25A and 25B illustrate the scenario when the ground faultdetermination occurs at or near a zero crossing of the output 32. FIG.25A illustrates a chart 38 with output 32. At time T_(f) a ground faultoccurred at the load. The output 32 is controlled to zero in response tothe ground fault. However, simply turning off all of the switches mayresult in ringing in the output filter (e.g., through the capacitor 21,the inductor 22, and/or the inductor 23). Instead, the output 32 iscontrolled to zero by adding pulse 37 to the PWM signal that controlsone or more of the switches of the inverter.

FIG. 26A illustrates an example response by actively controllingswitches of the inverter. The plot 131 shows the power provided to theoutput filter through the inverter switches SW1, SW2, SW3, and SW4 fromthe DC link. This power is normally transferred to the load.Specifically, the capacitor 21 is discharging to the load and chargingthrough the inductors 22, 23. When the plot 131 goes negative at t_(f),power is provided into the inverter circuit. Specifically, the capacitor21 is discharging both through the load or ground fault and through theinverter circuit to the DC link. It should be noted that the switchesSW1, SW2, SW3, and SW4 may be disabled after the output voltage hasreached zero.

FIG. 26B illustrates an example with the switches of the inverter turnedoff. When the switches are turned off at t_(f), power is not taken outof the capacitor 21 and the DC link. Any energy that is stored withincapacitor 21 must discharge through a load or ground fault on the outputof the inverter. With all switches turned off, no other current path ispresent to discharge capacitor 21.

FIG. 27 illustrates a controlled example using ground faultminimization. FIG. 27 includes an example plot 241 of voltage, anexample plot of current 242 through the output filter (e.g., inductor 22or 23), and an example plot 243 of current to the load. At the time ofground fault detection, t_(f), the switches are controlled to apply areverse voltage to the output filter. This reverse voltage induces theinductor current 242 to reverse, thereby reducing the amount of storedenergy from capacitor 21 that is delivered via the load current 243 tothe load and/or ground fault.

FIG. 28 illustrates an uncontrolled example without ground faultminimization. FIG. 28 illustrates an example plot 241 of voltage, anexample plot of current 242 through the output filter (e.g., inductor 22or 23), and an example plot 243 of current to the load. At the time ofground fault detection, t_(f), the switches are opened, disallowingcurrent to flow between the DC link and the output filter. The inductorcurrent 242 is interrupted and may be discharged through a snubbernetwork or similar mechanism. Without a path to remove energy from thecapacitor 21 to the DC link, the capacitive energy will dischargethrough the load and/or the ground fault. The magnitude of the loadcurrent 242 and duration from t_(f) to t_(z) will be dependent primarilyupon the energy in the capacitor 21 and the load and/or ground faultimpedance.

FIG. 29 illustrates a time plot of the power delivered to a load by aninverter containing an output filter with an inductive element and acapacitive element. The power output before the time t₀ is normal forthe inverter and is related to the load connected to the inverter whilea ground fault does not exist on the output of the inverter. The poweroutput after t₀ is provided partially to the load and partially to thefault e.g. the component, element, person or animal providing the groundfault path.

The dashed line 244 illustrates the power delivered to the load andfault during a ground fault scenario where the inverter disables allswitching elements. The power decreases from time t₀ to time t₂ where itis effectively 0. In this scenario, the area under the power time ploteffectively represents the energy stored in the output filter inductorand capacitor at time t₀.

The solid line 243 illustrates the power delivered to the load and faultduring a ground fault scenario where the inverter intentionally controlsthe output to 0. The power decreases from time t₀ to time t₁ where it iseffectively 0. In this scenario, the difference in the area under thepower time plots 243 and 244 effectively represents the differencebetween the energy stored in the output filter inductor and capacitor attime T0 and the energy intentionally transferred from the output filterinductor and output filter capacitor into the energy storage element orsupply of the inverter. For a time duration, a first energycorresponding to the switch signal after the ground fault signal is lessthan a second energy corresponding to a preceding switch signal for thesame time duration before the ground fault signal. It should be notedthat the energy related to solid line 143 is substantially less than theenergy related to dashed line 144.

It should be noted that the inverter typically changes from a minimumpower to a maximum power twice per cycle of the inverter output, oncefor the positive-going portion and once for the negative-going portion.In order to provide the appropriate output signal, an inverter may berequired to increase to a maximum power state and to reduce from thatmaximum power state to a zero-power state twice in a cycle of theoutput. In this case, the inverter will typically be capable of reducingfrom a maximum power state to a minimum power state in one quarter of anoutput cycle. If the inverter is able to actively control the outputfrom a maximum power state to a zero-power state during normaloperation, it should be able to actively control the output to azero-power state in a similar time in the event of a ground fault. Whennot controlled, the time to reduce to a zero-power state may be largelydependent on the characteristics of the load, with lower loads reducingthe output voltage more slowly.

FIG. 30 depicts a typical magnetization curve for an example magneticmaterial. The curves are plotted relative to current passing through acoil of a known number of turns placed in a way to enclose some or allof the magnetic material, resulting in an inductive element, such as aninductor or transformer.

The dashed line 245 illustrates the relative inductance, resistance tochanges in current, of an inductive element as compared to a nominalinductance at 0.1 T over a range of inductor currents. The relativeinductance of the inductive element can be seen to decreasesignificantly above a saturation knee current level i₀ and to decreaseto 10% of nominal inductance at a fully saturated current level i₁.

The solid line 246 illustrates the magnetic flux density containedwithin the example magnetic material of the inductive element over arange of inductor currents. The energy contained within the inductor islargely contained in the magnetic flux, which is directly related tomagnetic flux density in a material with a fixed or substantially fixedgeometry. At a current level i₀, the magnetic flux density within themagnetic material is approaching a reasonable maximum. At a currentlevel i₁, the magnetic flux density has reached a reasonable maximumlevel, where very large currents will be required to make a noticeableincrease in the magnetic flux density within the material.

The dashed line 245 is related to the slope of the solid line 246, asthe inductance of the inductive element is largely related to the changein flux density which influences the energy stored within the magneticmaterial.

Other magnetic materials may exhibit similar or different properties,but limiting the DC current maximum while providing maximum RMS outputcurrent is still beneficial for all materials with non-lineardiminishing behavior in the inductance (saturation characteristics).Changing air gap, number of turns, and material quantity may also changethe shape and behavior of the magnetization curve, but will retainsaturation behavior, even if this point occurs at a different currentlevel and with differently-shaped curves.

An inverter may rely on the inductance of the inductive element of theoutput filter to limit the rate that current increases into a faultcondition. If the inductance of the inductive element of the outputfilter is below a certain threshold, e.g. 100 uH, the output current mayincrease too quickly, resulting in excessive current on a switchingelement in the inverter e.g. 40A within a single switching cycle of theinverter e.g. 10 microseconds. To prevent damage to the inverter, theinductive element of the filter may be required to retain inductanceover this threshold, which may require increasing the size of theinductive element in order to support short-circuit current.

In order to provide sufficient current to allow protective devices suchas fuses or circuit breakers downstream of a power source to operate inan acceptable timeframe, a power source may be required to provide ahigher current to a short circuit condition than it provides to a normalload. In some cases, this current may be a multiple of the inverterrated current, such as three times rated current. Due to the lowimpedance of a short circuit the output voltage of the power source maybe reduced, potentially reducing the real power requirement of the load.In this case, the load on a power source feeding an inverter may bereduced, but the inverter may still need to supply a current to theshort circuit in order to operate downstream circuit protection devices.

FIG. 31 provides an illustration of the voltage of an internal energystorage element (illustrated as dashed line 251) and output current(illustrated as solid line 253) for an inverter where the voltage isreduced during a short circuit condition. The ripple on the outputcurrent of an inverter may be reduced by both the inductive element andthe capacitive element in the output filter of the inverter duringnormal operating conditions, but may only be reduced by the inductiveelement during a short circuit condition due to a reduction in theoutput voltage due to a low impedance connection across the output ofthe inverter. In the short-circuit condition, the output current ripplemay exceed a certain threshold (e.g. 1%) at the switch frequency of theinverter.

The ripple on the inductive element of the output filter is linked tothe voltage applied across the inductive element of the output filter,which may be related to the switching sequence of the inverter, thevoltage on the internal storage elements within the inverter, thevoltage across the capacitive element of the output filter, or acombination thereof. In some cases, reducing the operating voltage onthe internal storage elements within the inverter may reduce the rippleacross the inductive element of the output filter, especially caseswhere the output of the inverter is connected to a short circuit.

FIG. 32 provides a comparison between two different output waveshapes toproduce a comparable RMS (root mean squared) output. The peak currentrequired for the sinusoidal output is noticeably higher than the peakcurrent required for a square-wave output, while providing equivalentclearing current (for tripping of coordination devices, such as fusesand breakers). Lower peak currents may reduce required inductor size onan output filter of an inverter while providing protection for switcheswithin the inverter.

In some applications, the output current may be controlled not to exceeda current threshold relating to a saturation point of the inductiveelement of the output filter. Control to a maximum output current mayresult in a square wave output. Control to a maximum output current maybe performed based on a computed, estimated or preconfigured expectedrise rate of current in the inductive element of the output filter.

FIGS. 33-37 illustrate various embodiments of controlling the output ofthe inverter circuit to zero. In some embodiments, a controller 40receives data indicative of a ground fault and controls the switches ofthe inverter in response to the ground fault. In other embodiments, theinverter circuit includes logic to control the switches in response to aground fault signal.

FIG. 33 illustrates an example circuit for ground fault minimizationdetected by resistive impedance. The output connected to the loadincludes one or more resistors 43 connected to ground. The resistors 43may alternatively represent one or more components or devices in theload circuit that have a resistive property.

FIG. 34 illustrates an example circuit for ground fault minimizationdetected by capacitive impedance. The output connected to the loadincludes one or more capacitors 41 connected to ground. The capacitors41 may alternatively represent one or more components or devices in theload circuit that have a capacitive property. Alternatively, the outputfilter capacitor 21 may be connected to ground in such a way to providecapacitance 41.

A sensor, such as voltmeter 42, may detect the impedance to ground atthe output of the inverter circuit 11. The sensor may be internal orexternal to the controller 40, which analyzes the sensor data from thesensor. The controller 40 compares the impedance to a predeterminedthreshold. When the impedance to ground is less than the predeterminedthreshold, the controller 40 determines that a ground fault hasoccurred. In response to the determination of the ground fault detectedfrom the impedance seen at the output, the controller 40 generates acontrol signal for the switches of the inverter to cause the output 32to go to zero within the time delay. The time delay may be anintentional minimum delay or another preconfigured or computed delay.

FIG. 35A illustrates an example system with ground fault detected at aload circuit 52 connected to an inverter 51 implementing any of theembodiments for ground fault minimization. As described with respect toFIGS. 33 and 34, the ground fault may be detected at the inverter 51.FIG. 35B illustrates an example system with ground fault detected at aload circuit coupled with an engine generator set including engine 45and alternator 47.

The engine 45 may include one cylinder, two cylinders or another numberof cylinders. The one or more cylinders may include oscillating one ormore pistons. The one or more pistons may be guided through the one ormore cylinders by a connecting rod that is connected to a crankshaft bya crankpin. A combustion chamber includes a combustion chamber adjacentto a head of the piston. The combustion chamber is formed in a cylinderhead. In one phase of a combustion cycle for the piston, the exhaustport is blocked from the combustion chamber by the piston, and in asubsequent phase, the exhaust port is in gaseous connection with thecombustion chamber to release exhaust gas through the exhaust port to amuffler.

The alternator 47 is configured to generate a single-phase or poly-phasesignal through operation of the engine 45. The controlled fieldalternator 47 may include an exciter armature for generating a fieldcurrent. As the exciter armature is rotated in a magnetic flux, a timevarying voltage is induced in the windings of the exciter armature. Theoutput from the exciter armature is connected to the main field portionof generator. The connection may be made with or without brushes andslip rings. The field current of the output of the exciter provides amagnetic field in rotor field of the generator. As the field portion ofthe alternator is rotated relative to the stator, a magnetic flux ispassed through and across the alternator stator windings producing timevarying voltage. The field current from the exciter armature output maybe rectified or otherwise controlled.

The output of the alternator 47 may be a three phase signal. The phasesof the poly-phase signal may be offset one another by a predeterminedangle (e.g., 120 degrees or 2*Pi/3 radians). The poly-phase signal mayvary with respect to amplitude and frequency.

FIG. 36A illustrates an example system for ground fault detection at aload circuit 52. FIG. 36B illustrates an example system for ground faultdetection at a load circuit 52 coupled with an engine generator setincluding engine 45 and alternator 47. The load circuit 52 may includean external ground fault detector, such as a ground fault circuitinterrupter (GFCI), sensitive earth fault protection (5ONs), or groundfault alarm (GFA) that is configured to generate the ground faultsignal. The ground fault detector may compare the amount of currentflowing to the load circuit 52 to a threshold. When the external groundfault detector determines that current greater than the threshold isflowing in the load circuit 52, the ground fault signal is returned tothe inverter circuit 51.

FIG. 37A illustrates an example system with a controller 40 for groundfault detection at a load circuit 52. The controller 40 may receivesensor data collected at the load circuit 52 and analyze the sensor datato determine when the ground fault has occurred.

FIG. 37B illustrates an example system with a controller 40 for groundfault detection at an inverter circuit 51. The controller 40 may receivesensor data collected at the load circuit 52 and process the sensor datafor analysis at the inverter 51. For example, the controller 40 maysample the sensor data, average the sensor data, or perform anotherstatistical manipulation of the sensor data and send a value to theinverter circuit 51. The inverter circuit 51 compares the value to aground fault threshold and identifies the ground fault in response tothe value.

The controller 40 in any of these examples may control the inverterswitches to discharge one or more active components (e.g., capacitor 21)in the output filter of the inverter circuit.

The controller 40 may calculate a reverse voltage or a reverse current(reverse signal) based on the current state of the inverter switches.For example, the controller 40 may turn the switches on and off togenerate the predetermined output. For example, a sinusoidal signal maybe generated by applying a positive pair of switches (e.g., SW1 and SW4)in an increasing duration to reach a peak of the sinusoidal signal, thenapplying a negative pair of switches (e.g., SW2 and SW3) in anincreasing during to control the sinusoidal signal from the peak to thetrough. As an alternative to this sequence, various implementations arepossible to create the output signal based on different combinations ofthe inverter switches in a sequence. The controller 40 generates thereverse voltage or reverse current according to where in the sequencethe controller 40 is currently controlling the switches. The switchsequence may vary with varying load.

In one example, the controller 40 determines whether the negative pairof switches or the positive pair of switches are being energized. Whenthe negative pair of switches is being energized, the controller 40provides a negative pulse for the reverse signal. When the positive pairof switches is being energized, the controller 40 provides a positivepulse for the reverse signal. In this example, the polarity is changedbetween the current signal to the inverter switches and the reversesignal for the ground fault.

In another example, the controller 40 determines where the output is onthe output cycle. The controller 40 may make this determination based onthe slope of the output signal. The slope may be determined by adiscrete derivative of the output signal. Thus, in some examples, whenthe output is falling or rising near zero, the switch signal is notreversed but rather increased. For example, when the slope is negativebut the output is positive, the controller 40 provides a negative pulsefor the reverse signal. When the slope is positive but the output isnegative, the controller 40 provides a positive pulse for the reversesignal. When both the slope and output are positive or both negative,the controller 40 provides the opposite polarity for the reverse signal.

In another example, the controller 40 determines where the output is onthe output cycle. The controller 40 may make this determination based onthe time of the output signal. The time may be the amount of time sincethe last zero crossing, positive to negative zero crossing, or negativeto positive zero crossing. In one example, when less than apredetermined amount of time has passed since the zero crossing, noreverse signal is provided. Instead, the inverter switches are switchedoff. In another example, when the ground fault signal is received lessthan a predetermined time away from a zero crossing, a reduced reversalsignal. In another example, when the ground fault signal is receivedless than a predetermined time away from a negative zero crossing, apositive reverse signal is provided, and when the ground fault signal isreceived less than a predetermined time away from a positive zerocrossing, a negative reverse signal is provided.

In another example, the controller 40 generates the reverse signal basedon a trigger from the ground fault and according to a pulse width fromthe PWM signal. For example, the controller 40 may generate the PWMsignal for controlling the output of the inverter circuit such that thePWM signal includes varying sized pulses. Longer pulses increase thestored energy in the output filter, which is reflected in increasingoutput of the inverter circuit. The ground fault indication may be atrigger identified by the controller 40 to determine the size of thelast pulse in the PWM signal. The controller 40 determines a reversesignal that is proportional to the determined last pulse. That is largerrecent PWM pulses indicate more energy stored in the output filter andaccordingly a reverse signal with a larger parameter is used. Theparameter for the reverse signal may include any or a combination ofamplitude, duration or time. The controller 40 may include a lookuptable that associated pulse width values and reverse signal parameters.

The controller 40 may turn off the reverse signal and/or all controlsignals from the switches in response to the end of the determinedduration of the reverse signal. The determined duration may be set by apredetermined time such as a quarter wavelength of the output of thecircuit 11. The determined duration may be set according to a measuredelectrical parameter at the capacitor 21 or other component of theoutput filter. The electrical parameter may be voltage, charge, orcurrent.

In addition or in the alternative, the controller 40 may send a stopsignal to an interrupter device. The interrupter device may beconfigured to break the path of the DC link 20, which removes the sourceof the circuit 11. The interrupter device may break the path between thecircuit 11 and the load. The interrupter device may disable a boostconverter between a battery, fuel cell, or other source and the DC linkconnected to the circuit 11.

Referring back to FIG. 19, the controller 24 is an exampleimplementation of controller 40 of FIGS. 33, 34, 37A and/or 37B. Thecontroller 24 may include a processor 300, a memory 352, and acommunication interface 303. The controller 24 may be connected to aworkstation or another external device (e.g., control panel) and/or adatabase for receiving user inputs, system characteristics, and any ofthe values described herein. Optionally, the controller 24 may includean input device 304 and/or a sensor array 311. The sensor array 311receives sensor measurements from as described above (e.g., loadimpedance, inverter output, etc.). Additional, different, or fewercomponents may be included. The processor 300 is configured to performinstructions stored in memory 352 for executing the algorithms describedherein. The processor 300 may be compatible with a variety of engine andalternator combination and may identify an engine type, make, or model,and may look up system characteristics, settings, or profiles based onthe identified engine type, make, or model. Additional, different, orfewer components may be included.

FIG. 38 illustrates an example flow chart for the system for groundfault detection. The acts of FIG. 38 may be performed by the invertercircuit 51 for embodiments in which the controller 24 is internal to theinverter circuit 51 or alternatively by an external controller forembodiments in which the controller is external to the inverter circuit51. Additional, different of fewer acts may be included.

At act S201, the processor 300 determines an inverter output. Theprocessor 300 accesses from memory 352 or from real time measurement(e.g., sensing circuit or sensor array 311), a measured electricalquantity at an inverter output. The inverter output may be an actualpower signal applied to a load under a specification. The specificationmay be a target value for a sinusoidal signal at time intervals.Alternatively, the target value may specify an amplitude range or rootmean squared range for the inverter output. The target value may specifya variance or quality (e.g., THD) level for the inverter output.

At act S203, the processor 300 provides the output signal from theinverter circuit 51 to the load circuit 52. The processor 300 maygenerate a control signal (e.g., PWM signal) for multiple switches. Thecontrol signal may include a first PWM signal for a first pair ofswitches and a second PWM signal for a second pair of switches. Theinverter circuit 51, in response to the control signal, defines theelectrical parameters of the output signal that is provided to the loadcircuit 52.

At act S205, the processor 300 receives a ground fault signal. Theground fault signal may be detected at the inverter circuit 51 through ameasurement of the load. For example, the controller 24 or a sensorcoupled to the controller 24 monitors the impedance seen through theoutput terminals. The ground fault signal may be detected at an externalcontroller or at the load circuit 52.

At act S207, the processor 300 generates a switch signal for theinverter circuit in response to the ground fault signal. The switchsignal includes commands for the inverter switches. In one example, theswitch signal includes time duration settings or amplitude settings fortwo of the switches that form a path through the inverter circuit. Inone example, the switch signal includes time duration settings oramplitude settings for all of the switches in the inverter circuit. Theswitch signal generate in response to the ground fault signal maydesignate certain switch based on the switch settings before the groundfault. For example, when there are two pairs of switches, if a firstpair of switches is energized under normal operation before the groundfault is detected, a second pair of switches is energized by the switchsignal in response to the ground fault.

The processor 300 may include a general processor, digital signalprocessor, an application specific integrated circuit (ASIC), fieldprogrammable gate array (FPGA), analog circuit, digital circuit,combinations thereof, or other now known or later developed processor.The processor 300 may be a single device or combinations of devices,such as associated with a network, distributed processing, or cloudcomputing.

The memory 352 may be a volatile memory or a non-volatile memory. Thememory 352 may include one or more of a read only memory (ROM), randomaccess memory (RAM), a flash memory, an electronic erasable program readonly memory (EEPROM), or other type of memory. The memory 352 may beremovable from the network device, such as a secure digital (SD) memorycard.

In addition to ingress ports and egress ports, the communicationinterface 303 may include any operable connection. An operableconnection may be one in which signals, physical communications, and/orlogical communications may be sent and/or received. An operableconnection may include a physical interface, an electrical interface,and/or a data interface.

The communication interface 303 may be connected to a network. Thenetwork may include wired networks (e.g., Ethernet), wireless networks,or combinations thereof. The wireless network may be a cellulartelephone network, an 802.11, 802.16, 802.20, or WiMax network. Further,the network may be a public network, such as the Internet, a privatenetwork, such as an intranet, or combinations thereof, and may utilize avariety of networking protocols now available or later developedincluding, but not limited to TCP/IP based networking protocols.

While the computer-readable medium (e.g., memory 352 or database) isshown to be a single medium, the term “computer-readable medium”includes a single medium or multiple media, such as a centralized ordistributed database, and/or associated caches and servers that storeone or more sets of instructions. The term “computer-readable medium”shall also include any medium that is capable of storing, encoding orcarrying a set of instructions for execution by a processor or thatcause a computer system to perform any one or more of the methods oroperations disclosed herein.

In a particular non-limiting, exemplary embodiment, thecomputer-readable medium can include a solid-state memory such as amemory card or other package that houses one or more non-volatileread-only memories. Further, the computer-readable medium can be arandom access memory or other volatile re-writable memory. Additionally,the computer-readable medium can include a magneto-optical or opticalmedium, such as a disk or tapes or other storage device to capturecarrier wave signals such as a signal communicated over a transmissionmedium. A digital file attachment to an e-mail or other self-containedinformation archive or set of archives may be considered a distributionmedium that is a tangible storage medium. Accordingly, the disclosure isconsidered to include any one or more of a computer-readable medium or adistribution medium and other equivalents and successor media, in whichdata or instructions may be stored. The computer-readable medium may benon-transitory, which includes all tangible computer-readable media.

In an alternative embodiment, dedicated hardware implementations, suchas application specific integrated circuits, programmable logic arraysand other hardware devices, can be constructed to implement one or moreof the methods described herein. Applications that may include theapparatus and systems of various embodiments can broadly include avariety of electronic and computer systems. One or more embodimentsdescribed herein may implement functions using two or more specificinterconnected hardware modules or devices with related control and datasignals that can be communicated between and through the modules, or asportions of an application-specific integrated circuit. Accordingly, thepresent system encompasses software, firmware, and hardwareimplementations.

One or more embodiments may include an inverter comprising: an invertercontroller configured to operate a plurality of switches to provide analternating current to an output; and at least one surge protectiondevice for limiting a voltage on a direct current link of the inverter,the at least one surge protection device comprising: a variableresistance component coupled to the direct current link of the inverter;a sensing resistor selectively connected in parallel to the variableresistance component by a switching component; a detection deviceconfigured to measure a parameter of the sensing resistor; and a controlcircuit including failure sensing logic configured to detect a failurein the variable resistance component in response to information from thevariable resistance component and information from the detection devicefor the parameter of the sensing resistor.

One or more embodiments may include an inverter wherein the variableresistance component is a first variable resistance component, thesensing resistor is a first sensing resistor, the switching component isa first switching component, and the detection device is a firstdetection device, inverter further comprising: a second variableresistance component coupled to the direct current link of the inverter;a second sensing resistor selectively connected in parallel to thesecond variable resistance component by a second switching component;and a second detection device configured to measure a voltage across thesecond sensing resistor.

One or more embodiments may include an inverter, wherein the failuresensing logic analyzes information from the first detection device andthe second detection device to determine a conduction voltage for thefirst variable resistance component and the second variable resistancecomponent. One or more embodiments may include an inverter comprising adirect current link voltage sensor coupled to the direct current link ofthe inverter. One or more embodiments may include an inverter whereinthe failure sensing logic is configured to selectively connect a secondswitching resistor using the second switching component and calculate aconduction voltage for the first variable resistance component as adifference between the voltage across the second sensing resistor andthe output of the direct current link voltage sensor. One or moreembodiments may include an inverter, wherein the control circuit isconfigured to compare the conduction voltage for the first variableresistance component to an open circuit threshold and determine an openvariable resistance element or open condition when the conductionvoltage exceeds the threshold. One or more embodiments may include aninverter, wherein the control circuit is configured to activate afailure indicator when the conduction voltage exceeds the threshold. Oneor more embodiments may include an inverter, wherein the control circuitis configured to compare the conduction voltage for the first variableresistance component to a shorted range and determine shorted variableresistance element or open condition when the conduction voltage iswithin the shorted range. One or more embodiments may include aninverter, wherein the control circuit is configured to activate afailure indicator when the conduction voltage is within the shortedrange. One or more embodiments may include an inverter, wherein thefailure indicator includes a message to a management device. One or moreembodiments may include an inverter comprising a fuse connected to thefirst variable resistance component, wherein the control circuit isconfigured to determine the fuse is blown in response to data from thedirect current link voltage sensor. One or more embodiments may includean inverter wherein the control circuit is configured to activate afailure indicator when the fuse is blown. One or more embodiments mayinclude an inverter wherein the variable resistance component includes ametal oxide varistor (MOV), a gas discharge tube, or a Zener diode. Oneor more embodiments may include an inverter, wherein the control circuitgenerates a control signal for operation of a semiconductor component,in parallel with the variable resistance component, in response to anoutput of a direct current link voltage sensor.

One or more embodiments may include a method for limiting a voltage on adirect current link of an inverter with a surge suppression deviceincluding a variable impedance component and a sensing component, themethod comprising: receiving measurement data from a voltage detectiondevice configured to measure a voltage across the sensing component;performing an analysis of the measurement data for the voltage acrossthe sensing component; determining a conduction voltage for the variableimpedance component based on the analysis; performing a comparison ofthe conduction voltage and a threshold; and signaling a failure for theinverter in response to the comparison. The method may include receivingsecond measurement data for the direct current link of the inverter,wherein the analysis includes the second measurement data.

One or more embodiments may include a system including an invertercontroller configured to operate a plurality of switches to provide analternating current to an output; and a plurality of surge protectiondevices for limiting a voltage on a direct current link of an inverter,each surge protection device comprising: a plurality of variableimpedance components coupled to the direct current link of the inverter;a plurality of sensing components selectively connected in parallel tothe plurality of variable impedance components; one or more detectiondevices configured to measure a parameter of the plurality of sensingcomponents; and a control circuit including failure sensing logicconfigured to detect a failure in the one of the plurality of variableimpedance components in response to information from the one or moredetection devices for the parameter. The system may include an indicatorconfigured to provide a user with an indication of the failure, whereinthe inverter controller is configured to disconnect the inverter from apower grid in response to the failure and/or wherein the invertercontroller is configured to generate a notification to replace one ofthe plurality of surge protection devices in response to the failure.

One or more embodiments may include an inverter system having aninverter controller and at least one surge protection device. Theinverter controller is configured to operate multiple switches toprovide an alternating current to an output. The at least one surgeprotection device is configured to limit a voltage on a direct currentlink of the inverter. The at least one surge protection device includesa variable resistance component coupled to the direct current link ofthe inverter, a sensing resistor selectively connected in parallel tothe variable resistance component by a switching component, a detectiondevice configured to measure a parameter of the sensing resistor, and acontrol circuit including failure sensing logic configured to detect afailure in the variable resistance component in response to informationfrom the variable resistance component and information from thedetection device for the parameter of the sensing resistor.

One or more embodiments may include a circuit for minimizing energyprovided to a ground fault, the circuit comprising: a source; aplurality of switches including a first side pair of switches and asecond side pair of switches configured to provide an output signalbased on the source; an output filter including one or more energystorage elements coupled to the first side pair of switches or thesecond side pair of switches; and a controller configured to receive aground fault signal that indicates a fault has occurred and configuredto generate a switch signal for the plurality of switches for a minimumenergy state of the output filter and in response to the ground faultsignal. The one or more energy storage elements includes: a firstinductor coupled to at least the first side pair of switches of theplurality of switches, a second inductor coupled to at least the secondside pair of switches of the plurality of switches, and at least onecapacitor coupled to the first inductor and the second inductor for anoutput of the circuit for ground fault correction, wherein the switchsignal in response to the ground fault signal changes a state of atleast one of the plurality of switches, wherein the at least one of theplurality of switches includes the first side pair of switches and thesecond side pair of switches, and/or wherein the switch signal inresponse to the ground fault signal reverses a plurality of the outputsof the circuit for ground fault correction. The ground fault signal maybe generated by an external ground fault detector and/or detected froman impedance from the output filter to ground, wherein the impedance isa capacitive impedance or a resistive impedance. The switch signal maybe configured to change the plurality of switches to control the outputsignal to zero voltage, wherein a time elapsed between a receipt of theground fault signal at the output signal reaching substantially zerovoltage is less than a quarter cycle of the output signal. The switchsignal may include a first pulse width modulated signal for the firstside pair of switches and a second pulse width modulated signal for thesecond side pair of switches, wherein energy is actively removed fromthe output filter and stored internal to an inverter including thecircuit after a ground fault signal is received.

One or more embodiments may include a method for ground fault correctioncomprising: providing an output signal via a plurality of switchesincluding a first side pair of switches coupled to a first inductor anda second side pair of switches coupled to a second inductor; receiving,at a controller, a ground fault signal that indicates a fault hasoccurred; and generating a switch signal for the plurality of switchesin response to the ground fault signal, the switch signal activelycontrolling the output signal to a threshold, wherein the output signalis a voltage and the threshold is zero, and/or wherein the switch signalin response to the ground fault signal reverses a plurality of theoutputs for ground fault correction. The ground fault signal may begenerated by an external ground fault detector and/or detected from animpendence from an output filter to ground.

One or more embodiments may include an inverter circuit including aninput and an output; a ground fault sensor coupled to the invertercircuit and configured to generate a fault signal that indicates aground fault has occurred; a controller configured to receive the groundfault signal and generate a reverse signal for operation of the invertercircuit that reverses energy from the output of the inverter circuit tothe input of the inverter circuit, and a plurality of switches in theinverter circuit including a first side pair of switches and a secondside pair of switches configured to provide the output, wherein thereverse signal changes a state of the first side pair of switches or thesecond side pair of switches.

One or more embodiments may include a circuit for minimizing energyprovided to a ground fault includes a source, a multiple switches, anoutput filter, and a controller. The switches include a first side pairof switches and a second side pair of switches configured to provide anoutput signal based on the source. The output filter includes one ormore energy storage elements coupled to the first side pair of switchesor the second side pair of switches. The controller is configured toreceive a ground fault signal that indicates a fault has occurred andconfigured to generate a switch signal for the switches for a minimumenergy state of the output filter and in response to the ground faultsignal.

We claim:
 1. An inverter comprising: an inverter controller configuredto operate a plurality of switches to provide an alternating current toan output; and at least one surge protection device for limiting avoltage on a direct current link of the inverter, the at least one surgeprotection device comprising: a variable resistance component coupled tothe direct current link of the inverter; a sensing resistor selectivelyconnected in parallel to the variable resistance component by aswitching component; a detection device configured to measure aparameter of the sensing resistor; and a control circuit includingfailure sensing logic configured to detect a failure in the variableresistance component in response to information from the variableresistance component and information from the detection device for theparameter of the sensing resistor.
 2. The inverter of claim 1, whereinthe variable resistance component is a first variable resistancecomponent, the sensing resistor is a first sensing resistor, theswitching component is a first switching component, and the detectiondevice is a first detection device, inverter further comprising: asecond variable resistance component coupled to the direct current linkof the inverter; a second sensing resistor selectively connected inparallel to the second variable resistance component by a secondswitching component; and a second detection device configured to measurea voltage across the second sensing resistor.
 3. The inverter of claim2, wherein the failure sensing logic analyzes information from the firstdetection device and the second detection device to determine aconduction voltage for the first variable resistance component and thesecond variable resistance component.
 4. The inverter of claim 2,further comprising: a direct current link voltage sensor coupled to thedirect current link of the inverter.
 5. The inverter of claim 4, whereinthe failure sensing logic is configured to selectively connect a secondswitching resistor using the second switching component and calculate aconduction voltage for the first variable resistance component as adifference between the voltage across the second sensing resistor andthe output of the direct current link voltage sensor.
 6. The inverter ofclaim 5, wherein the control circuit is configured to compare theconduction voltage for the first variable resistance component to anopen circuit threshold and determine an open variable resistance elementor open condition when the conduction voltage exceeds the threshold. 7.The inverter of claim 6, wherein the control circuit is configured toactivate a failure indicator when the conduction voltage exceeds thethreshold.
 8. The inverter of claim 5, wherein the control circuit isconfigured to compare the conduction voltage for the first variableresistance component to a shorted range and determine shorted variableresistance element or open condition when the conduction voltage iswithin the shorted range.
 9. The inverter of claim 8, wherein thecontrol circuit is configured to activate a failure indicator when theconduction voltage is within the shorted range.
 10. The inverter ofclaim 9, wherein the failure indicator includes a message to amanagement device.
 11. The inverter of claim 4, further comprising: afuse connected to the first variable resistance component, wherein thecontrol circuit is configured to determine the fuse is blown in responseto data from the direct current link voltage sensor.
 12. The inverter ofclaim 11, wherein the control circuit is configured to activate afailure indicator when the fuse is blown.
 13. The inverter of claim 1,wherein the variable resistance component includes a metal oxidevaristor (MOV), a gas discharge tube, or a Zener diode.
 14. The inverterof claim 1, wherein the control circuit generates a control signal foroperation of a semiconductor component, in parallel with the variableresistance component, in response to an output of a direct current linkvoltage sensor.
 15. A method for limiting a voltage on a direct currentlink of an inverter with a surge suppression device including a variableimpedance component and a sensing component, the method comprising:receiving measurement data from a voltage detection device configured tomeasure a voltage across the sensing component; performing an analysisof the measurement data for the voltage across the sensing component;determining a conduction voltage for the variable impedance componentbased on the analysis; performing a comparison of the conduction voltageand a threshold; and signaling a failure for the inverter in response tothe comparison.
 16. The method of claim 15, further comprising:receiving second measurement data for the direct current link of theinverter, wherein the analysis includes the second measurement data. 17.A system comprising: an inverter controller configured to operate aplurality of switches to provide an alternating current to an output;and a plurality of surge protection devices for limiting a voltage on adirect current link of an inverter, each surge protection devicecomprising: a plurality of variable impedance components coupled to thedirect current link of the inverter; a plurality of sensing componentsselectively connected in parallel to the plurality of variable impedancecomponents; one or more detection devices configured to measure aparameter of the plurality of sensing components; and a control circuitincluding failure sensing logic configured to detect a failure in theone of the plurality of variable impedance components in response toinformation from the one or more detection devices for the parameter.18. The system of claim 17, further comprising: an indicator configuredto provide a user with an indication of the failure.
 19. The system ofclaim 17, wherein the inverter controller is configured to disconnectthe inverter from a power grid in response to the failure.
 20. Thesystem of claim 17, wherein the inverter controller is configured togenerate a notification to replace one of the plurality of surgeprotection devices in response to the failure.